Left-foot braking is a technique for all sorts of motorsport. It's somewhat argued, but can improve lap times. It involves using the right foot to operate the throttle alone, while the left foot operates the brake, clutch and rest pedal. It can be used efficiently in any drive, but it is particularly beneficial in FWD, Front-Engine AWD and Mid-Engine RWD cars.
It is not recommended on road cars, especially for those inexperienced in it (to which it is generally not advised, unless in a Rally stage). The reasons being shocks due to unsmooth braking, damage to the drive train due to both acceleration and braking, and lost of body support by not operating the footrest with the left foot.
First off, we need to distinguish between left foot braking in two different purposes: The first being to reduce transition time as you go from pedal to pedal. The second is where the driver uses left foot braking to manage weight transfer and torque more sensitively (professionally called "Combined braking").
Left foot braking
reduction of pedal transition time
Increased Control over weight distribution and power
On the straight, usually before a curve that does not require downshifting
While cornering, usually on moderate speed corners
Racing, road driving (sometimes), Rallying
Racing (rare), Rallying, Drifting
Regardless of car setup, although beneficial to turbocharged vehicles
Most beneficial in a FWD
Another, more rare use of the left foot on the brake pedal is actually when accelerating on the straight, in order to check and see if the brakes are still responsive and work effectively.
combined braking has the following advantages:
Reduction of understeer before the APEX: With left foot braking you can use trail braking through the turn-in point, while smoothly increasing throttle application through the corner. Ideally, you should be able to control weight transfers to enable the car to slip slightly without understeering or oversteering, but in a state of "neutral steering". This will actually optimize the traction of your tires. Also, should the car understeer due to power, you would be able to punch the brakes hard for a moment, which will straighten you up (lock rear tires).
Increase of power application after the APEX: oddly enough, covering the brakes with the left foot as you progressively accelerate out of corners will enable better acceleration with less wheelspin and more control. This is particularly beneficial with cars that have locking differentials. The reason is that a slight braking pressure is light enough to not decrease the power application, but does create a slight conflict which the differential "recognizes" this as the wheels slipping, resulting in it locking-up and giving better transmission of engine torque to the wheels, particularly in a FWD. This can also work in a RWD, but because of weight shifting involved, is harder to perform well.
Allowing for a better control and recovery from oversteer: Left-foot braking is also a method of inducing slides, controlling them once developed, intentional or not, and terminate them, all done far more efficiently. This is particualrly usefull in rallying or in drifting. Instead of simply using power to terminate a slide or brakes to initiate it, the driver can brake and accelerate to initate the slide, and than immediatly (no pedal transition time) accelerate and brake (the stress this time on accelerate) to power out of the slide. In a rear-wheel drive, punching the brake hard for a moment or so while on the accelerator, can terminate oversteer.
Left foot braking has the following advantages:
Removes pedal transition time: By left-foot braking, throttle and brake applications will in fact slightly overlap. This allows for a quicker slow down and power-out. This is particularly useful for WRC drivers, who drive in turbo-charged cars. In this case, peeling off of the throttle creates a greatly enriched mixture of fuel and air, that powers up the turbo-charger. Left-foot braking allows to power out quickly and use this extra boost, without letting the revs drop too low. When the car has no clutch, or when approaching a curve that does not require a downshift (And therefore no clutching), you should do this.
Allows to maintain brake integrity: When approaching a short braking zone at the end of a fast straigt, followed by a sharp or moderate-speed corner with little run-off areas, you should consider dabbing the brakes ever so slightly a bit before the braking zone, to check the feel of the brakes, in order to be aware of possible brake fade or pad-knockback. This should also be done after recovery from a aquaplaning situation or when after going through watersplashes.
Decrease chance of overtake: By rythemly dabbing the brakes ever so slightly when you are about to brake before a corner, you alert the opponent and also makes a beginner or a competetor unfimiliar to the track unaware of your percise braking point. This helps in avoiding being ovetaken over the straight before the corner. If you see brake lights flashing slightly a bit before the braking zone, or when coming out of corners, but no significant deceleration seems to take place, you should know that the driver in front is testing his brakes.
Left foot braking has the disadvantage of a less sensitive braking and lost of tactile feedback, because the left foot is not used to push the body back into the seat, thus hanging over the left foot. In a harnessed bucket seat it's not that serious of an issue, but still decreases feedback. Additionally, it can decrease power and/or induce oversteer, and create an uneven footwork. This last consideration depends on whether the driver needs to use a clutch. If not, left foot braking becomes far more recommended. It's also less recommended in a rear-wheel drive, however it is far from useless, even in a RWD. It is possible to Heel and Toe or tiptoe while left foot braking, but it's a skill which is hard to adopt. It's better to use the right foot on the straight and use the left foot according to the car, corner and speed.
Normally, in a typical race car, we recommend that the driver adopt left foot braking (Combined braking) while apexing through moderate-speed bends, that require a lot of accuracy. The driver should use Threshold braking with the right foot before the corner, Heel and Toe just as he comes to it, and than move the left foot to the brake in order to commit trail braking up to the (late) APEX.
Left foot braking (not combined) can be used to brake before a fast curve that does not require a downshift. Usually, there's no need to continue this drill through the curve, because the corner is best handled by breathing the throttle, especially because the speed does not enable generation of wheelspin.
Left foot braking is also good for Drifting, if the driver is well-balanced in the seat and if it can help his drifting style.
However, when left-foot braking into corners, you might be feeling as though if you are not braking, but only "covering" the brakes. This is because the left foot has the habit of pressing the clutch deep and quick, or simply squizzing on the rest pedal. That's why you need to be gentle and smooth.